The major goal of this competing renewal is to define viral interactions with host pathways that are essential for the assembly of infectious HIV-1 particles. HIV-1 particle assembly occurs on the plasma membrane of infected cells. A major finding from this project was the identification of Rab11-FIP1C (FIP1C) as a key adaptor protein required for HIV-1 envelope protein (Env) trafficking and particle incorporation. FIP1C-dependent trafficking underlies the cell type-specific incorporation of Env, as disruption of a key motif in the cytoplasmic tail (CT) involved in FIP1C translocation led to a loss of Env incorporation in cell types that was nearly identical to that of CT-deleted viruses. Our recent data establish that Env traverses the endosomal recycling compartment, where it meets FIP1C in a step required for subsequent outward trafficking and particle incorporation of relevant primary isolate Envs. Simian Immunodeficiency Virus (SIV) Envs and a subset of HIV Envs, in contrast, do not follow FIP1C-dependent trafficking in human cells. This competing renewal will fully define the viral and host determinants of Env trafficking pathways. Experiments here will identify additional members of the HIV-1 Env trafficking complex, including the essential kinesin mediating outward sorting of Env. Experiments in this application will define the sequential steps in Env trafficking that lead to the production of infectious viral particles, using a novel imaging strategy. FIP1C-mediated trafficking will be characterized in primary monocyte-derived macrophages, where membrane dynamics and the intracellular virus-containing compartment may introduce unique features. The essential role of FIP1C in the formation of the virological synapse and in macrophage-to-T cell transmission events will then be delineated.